Fix return of x509_self_test without SHA-1

No being able to run the test is not a failure
diff --git a/library/x509.c b/library/x509.c
index 0d42951..14ec322 100644
--- a/library/x509.c
+++ b/library/x509.c
@@ -1016,7 +1016,7 @@
     return( 0 );
 #else
     ((void) verbose);
-    return( MBEDTLS_ERR_X509_FEATURE_UNAVAILABLE );
+    return( 0 );
 #endif /* MBEDTLS_CERTS_C && MBEDTLS_SHA1_C */
 }